Electrical insulating oil



Patented Aug. 14, 1934 TATES ELECTRICAL INSULATING OE Seymour W. Ferris,Lansdowne, Pa., assignor to The Atlantic Refining Company, Philadelphia,Pa, a corporation of Pennsylvania No Drawing. Application .lanuary it,1931, Serial No. 568,004

7 Claims.

The present invention relates to improvements in mineral oilcompositions, and has particular reference to the production of improvedoils for use in connection with electrical equipment, as

an insulating and/or cooling medium.

' The reason for the use of insulating oils in electrical equipment suchas capacitors, transformers, and cables is primarily based upon the factthat these oils have a greater resistance to electrical potential thandoes air, for example. With different types of apparatus and undervarying conditions of use, insulating oils may have a range ofviscosities varying, for example, between 50 seconds Saybolt Universalat 100 F. and 200 seconds Saybolt Universal at 210 F. The value of anyparticular insulating oil depends upon its electrical qualities, inparticular, its resistance to the flow of an electric currenttherethrough. This quality is known in the art as resistivity, g0 and iscustomarily measured in meg. meg. ohms per cmfi, according to A. S. T.M. method D-l-291.

Electrical oils have been developed, in accordance with the prior art,which have a satisfactory initial resistivity; but upon being subjectedto use in electrical equipment over an extended period of time it ischaracteristic of the prior art oils that they lose their highresistivity or break down. In order to determine in advance theresistivity-stability of a particular oil or how long it will stand upin actual use in electrical equipment, there has been developed anoxidation test, which is accepted as approximating extended operatingconditions. This method consists in placing 100 gms. of the oil to betested into a 250-330 cc. Pyrex Erlenmeyer flask, closing the mouth ofthe flask with a cork stopper'perforated or slotted so that a freecirculation of air may be maintained over the oil, and immersing theflask in an oil bath maintained at 100 C., allowing it to remain in suchbath for '72 hours. this time the oil sample is removed from the bathand its resistivity measured at 85 C. in accordance with the A. S. T. M.method D-257-29T. A comparison of the results of this test with itsinitial resistivity constitues a means for evaluating theresistivity-stability of an oil. v

A primary object of the present invention is the production of oilswhich have a higher resistivity-stability than oils known heretofore,and more specifically oils which will not substantially decrease inresistivity or which will remain of substantially the same resistivityfor prolonged periods of time, or which may, in fact, after actual 55use and/or when subjected to the aforesaid test After show a finalresistivity greater than their, initial resistivity.

In my copending application, Serial Number 206,388 filed July 16, 1927,which on January 13,

1931, issued as U. S. Patent No. 1,788,569, of which the presentapplication is a continuation in part, there is described a process forseparating an oil into constituents which are respectively moreparafiinic and more naphthenic, by extraction of the latter type ofconstituents from the original oil with an aromatic nitro compound suchas nitrobenzene, The various series of hydrocarbons which comprisemineral oils possess a differential solubility in nitrobenzene,naphthenic compounds bein more soluble therein than parafi'iniccompounds.

I have found that by a modification of this process I am able to produceinsulating oils having novel characteristics with respect toresistivity-stability. A petroleum oil, for example, a refineddistillate, is divided into a number of. fractions by extraction withnitrobenzene. Each extraction is preferably made by heating one part ofoil with two parts of nitrobenzene to miscibility, cooling the mixtureto 10 0., whereby a two layer system is formed on settling, separatingthe layers, and removing nitrobenzene from each fraction. The operationis then repeated on the nitrobenzene undissolved fraction anydesired-number of-times. These .various fractions, particularly aftertreatment with fuller's earth, behave differently with respect toresistivity-stability. Certain of the fractions, for example, the firstdissolvedfraction will act somewhat as the insulating oils knownheretofore; that is, will show a pronounced decrease in resistivityafter being subjected to the aforesaid oxidation test or afterequivalent use in electrical apparatus. However, other fractions willact in a manner entirely unknown in the prior art,'that is, certain ofthese fractions will show an increase in resistivity upon actual use orwhen subjected to said oxidation test. The fraction or fractions whichhave this novel characteristic of increasing in resistivity will dependupon such factors as the particular oil and conditions under which theextraction and clay treatment are effected. My invention will be morespecifically explained in the following examples:

Example I A pipe still residuum from a Mid-Continent pressed paramndistillate, having been subjected 11 to acid treatment in the amount of14 lbs. of 66 B. H2804 per barrel of oil and a subsequent contact claytreatment, was employed as a starting material. This refined oil had aviscosity of 284 seconds Saybolt Universal at 100 F., and a gravity of250 A. P. I. 100 parts of the oil was heated with 50 parts ofnitrobenzene to complete miscibility. This mixture was then cooled to 10C. and settled, whereby a two layer system comprising a lower layer ofnitrobenzene dissolved oil and an upper layer of nitrobenzeneundissolved oil was formed. The two layers were separated, by decanting,and the two oil fractions freed from nitrobenzene by vacuumdistillation. 80.7 parts of nitrobenzene undissolved oil resulted andthis was heated to miscibility with 75 parts of nitrobenzene, and themixture treated as before. An additional extraction was made upon the 63parts of nitrobenzene undissolved fraction resulting from the secondextraction, 150 parts of nitrobenzene being employed. From this thirdextraction was recovered 30.7 parts of undissolved oil. After beingfreed from nitrobenzene the first, second and third nitrobenzenedissolved fractions and the final undissolved oilwere percolated throughfinely divided fullers earth at a temperature of approximately 70 C.Initial electrical resistivity tests were made upon each oil fraction,after which the fractions were subjected to the oxidation testaforementioned, and final resistivities taken. The results may betabulated as follows:

It will be noted that the nitrobenzene undissolved fraction No. 3 has aninitial electrical resistivity of 5.52 meg. meg. ohms per cm. whichincreases after being subjected to the oxidation test to 28.4 meg. meg.ohms per cm. This particular fraction makes a highly desirableinsulating oil for commercial use in electrical instruments. Theremaining fractions exhibit a drop in resistivity which ischaracteristic of insulating oils known heretofore.

Example II A distillate from an acid-treated Mid-Continent crude oil wasfiltered through fullers earth, to produce a refined. oil having aviscosity of 163 seconds Saybolt Universal at 100 F., and a specificgravity of 0.8905 at 15.5" C. This oil was subjected to two extractionswith nitrobenzene, in

each extraction employing two parts of nitrowere made. The results maybetabulated as follows:

Resistivity Yield 30 D cent vol.

Clay

After72 hours Origi- Ori inal 1st soluble fraction 2nd solublefraction..... Undissolved fraction- With this particular oil and underthe conditions of treatment, it is seen that the second nitrobenzenedissolved fraction exhibits an increase in electrical resistivity afterbeing subjected to the oxidation test.

Example III parts of a Pennsylvaniadistillate having a viscosity of 307seconds Saybolt Universal at 100 F., and a gravity of 29.4 A. P. I. wassubjected to eight extractions with nitrobenzene, each extraction beingmade by heating 200 parts of the solvent with the undissolved residuefrom the last extraction to obtain miscibility and cooling to effect theformation of a two-layer system. Dissolved fractions 4 and 5 were mixedas were 195 dissolved fractions 6, 7, and 8. After being freed fromnitrobenzene by vacuum distillation and clay filtered at 75 C., thefractions were subjected to the oxidation test, resistivitydeterminations being made before and after said test. uq

Resistivity Per- 53% s Vis. vis cent i grav. Q

by per 17 rist After 72 ton Init. hours Dissolved fraction 27.6 243.9213 648 .862 2.43 0.2 Dissolved fraction 16.0 243 .8833 321 .822 13.314.51% D1ssolvcd fraction 11.6 243 .8682 243 .816 52.3 37.8 Dissolvedfraction ,#5 20.8 243 .8592 223 .706 01.8 61.8 Dissolved fraction Aswill be recognized by one skilled in the art an oil which increases inelectrical resistivity upon being subjected to conditions which cause asubstantial decrease in the resistivity of oils known heretofore,constitutes a considerable improve- 13 ment in the art. I am unable toexplain why the oils produced according to my process exhibit this novelcharacteristic. Regardless of theory, however, I have found that theseoils are highly desirable for use as an insulating and/or cooling 13medium in electrical equipment.

While my invention has been described somewhat specifically, it isintended to include modifications coming within the spirit thereof andas defined by the appended claims.

What I claim is:

1. A process for producing insulating oils, which comprises separating aviscous petroleum oil into a number of fractions by extraction withnitrobenzene, removing the nitrobenzene from 14 said fractions,contacting said fractions with a solid adsorptive agent, thereafterremoving said agent therefrom, and selecting for use as insulating oils,those fractions, samples of which upon being subjected to a temperatureof 100 C. 1!

1...; 3163M. icei ieum oil into fractions by solvent extraction, ie-

moving the solvent from the soluble and insoluble fractions maintainingsamples of the fraciions at 2. temperature of 160 C. for 12 hours, andselecting those fractions, samples of which show no substantial decreasein eiectricei resistivity after being subjected to said temperature forsaid time, for use as insoleting oiis.

irsuieiing eii preserezi e3 minera oil e solver-1' selves; irom thezest-ii ins, seie. eii, upon me". ning 2. s pie ihereei e C. for F2hours, showing "'sntiei in electrical resistiv Fm ele ,ei insulating eiicomps. 5.1g a viseeus mineral oil heving, when measures. at 35 C. inaccordance with A. S. M. D257-29T, an electrics-i resistivity 0i least 5meg. meg. ohms per cmfi, which upon being maintained at e. eereiure of108 C. for 32 hours, exhibits suesiee. 2.113 no decrease in eieeiicslresistivity.

5. An electrical insulating oil 0 mprising a visceus mineral eil having,when measured at in accoidance with A. S. T. M. method D-257-2Q'i, anelectric-s1 resistivity of at least 5 meg. me. ohms per cmfi, which uponbeing maintained at a, temperature oi C. for 72 hours, exhibits anincrease in electrical resistivity.

SEYNKOUR W. FERRIS.

